Ferrite isolator utilizing aligned crystals with a specific anisotropy constant



6, 1963 E F R. A. SCHLOEMANN 3,100,288

FERRITE ISoLA'roR UTILIZING ALIGNED CRYSTALS WITH A SPECIFIC ANISOTROPYCONSTANT Filed Jan. 5, 1961 //V VE/V T 0/? ERNS T E R. A. SCHLOEMAN/VATTORNEY 3,100,288 FERRITE ISOLATOR UTILIZING ALIGNED CRYSTALS WITH ASPECIFIC ANISOTROPY CONSTANT Ernst F. R. A. Schloemann, Weston, Mass,assignor to Raytheon Company, Lexington, Mass, a corporation of DelawareFiled Jan. 5, 1961, Ser. No. 80,920

2 Claims. (Cl. 33.3-24.2) 1

This invention relates generally to ferrite devices and, moreparticularly, to the use of grain oriented ferrite materials in aresonance isolator device.

In resonance isolators, it is desirable to obtain a large ratio ofreverse attenuation to forward attenuation so that more effectiveisolation can be maintained in a microwave system. Large ratios havebeen attained in the past only at the sacrifice of bandwith. If thebandwidth B is defined as that frequency range over which the reverse toforward attenuation ratio R is at least onehalf of its maximum value, itcan he shown that the maximum reverse to forward ratio is inverselyproportional to the square of the bandwidth. It is, therefore,reasonable to define a figure of merit F of an isolator in accordancewith the following formula:

2 F max In the above formula F is the figure of merit, R is the reverseto forward attenuation ratio, B, is the bandwidth and w is the operatingfrequency of the device, that is the frequency for which R assumes itsmaximum value.

Bandwidth in a resonance isolator is essentially propontional to theferromagnetic resonance linewidth of the material used with a constantof proportionality that is a relatively sensitive function of the shapeof the ferrite material, its magnetization, the freqency of operation,and the cut-off frequency of the waveguide. Under most conditions thebandwidth is substantially smaller than the resonance linewidth. Thisinvention, however, provides a substantial increase in the bandwidthover that obtained in conventional isolators by utilizing a uniqueorientation of the crystals of the ferrite material in accordance withthe anisotropy characteristics of the material. The invention canprovide optimum bandwidth response for either single crystals or fororiented polycrystalline materials. In either case the crystals arealigned so that their magnetization vectors are forced to precess alongan ellipsoidal cone which has its major axis perpendicular to thewaveguide axis. No isolators known at the present time provide theincrease in bandwidth provided by this invention and at the same timeprovide a maximum reverse to forward attenuation ratio.

In a specific embodiment of the invention, for example, which utilizes apolycrystalline material made of hexagonal crystals, the polycrystallinematerial is grainoriented so that the hexagonal axes of the individualcrystals are substantially aligned. In order to assure correctorientation in the waveguide, the ferrite material is positioned in thewaveguide in such a manner that the aligned hexagonal axes of thepolycrystalline material are substantially parallel to and coincide withthe direction of the longitudinal axis of the waveguide.

The specific embodiments and the operation of the invention may be moreclearly described with reference to the following drawing wherein:

FIG. 1 shows a rectangular waveguide utilizing a ferrite material inaccordance with the invention;

FIG. 2 shows an alternative embodiment of a rectangular waveguideutilizing the invention; and

FIG. 3 shows the orientation of various crystalline structures toindicate the alignment of selected axes of ice the material with thelongitudinal axis of the Waveguide and with the direction of the D.-C.magnetic held.

In FIG. 1 there is shown a rectangular waveguide 10 having placedtherein a slab 11 of ferrite material. In this figure the ferrite slabis place so as to be perpendicular to the top and bottom walls 12 and13, respectively, of waveguide 10. FIG. 2 shows a Waveguide 10 havingplaced therein a ferrite slab 14 resting on the bottom wall 13 of thewaveguide. In FIGS. 1 and 2 magnetic fields are applied as shown byutilizing permanent magnets 15 so as to provide a magnetic field H in adirection perpendicular to the top and bottom walls of the guide, asshown by arrows 16. Arrows 17 show the direction of the longitudinalaxis of the waveguide.

By the correct orientation of a single crystal or of a polycrystallineoriented ferrite material, the figure of merit for an isolator usingsuch a material can be optimized so as to provide a maximum reverse toforward attenuation ratio and a maximum bandwidth. The orientation ofthe crystals should be such that the magnetization vector is forced toprecess along an ellipsoidal cone which has its major axis perpendicularto the waveguide axis.

If the crystal, for example, has a hexagonal crystalline structure, thisdesired effect can be achieved, provided the first order anisotropyconstant of the material is negative, if the crystals are orientedin'such a way that the hexagonal axis coincides with the waveguide1ongitudinal axis. This can be shown with the help of FIG. 3 and FIG.3a. In FIG. 3 there is shown diagrammatically a rectangular waveguidestructure 20 having a longitudinal axis direction 21 and a DC.magnetization field direction 22 as shown by arrows. If a materialhaving a hexagonal crystalline structure is used in accordance witheither of the configurations shown in FIGS. 1 and 2, it is necessarythat the hexagonal axis be aligned with the longitudinal axis 21 ofwaveguide 20. This alignment assures that the magnetization vector ofthe material precesses along an ellipsoidal cone which has its majoraxis perpendicular to the waveguide axis. FIG. 3a shows a piece ofhexagonal crystalline material 25 wherein the hexagonal crystals havebeen aligned within the material so that their hexagonal axes,represented by the arrow 26, are all substantially parallel to eachother.

The crystals as utilized in ferrite slabs 11 or 14 of FIGS. 1 and 2 arecut so that the crystals within the material have their internallyaligned hexagonal axes positioned so as to be substantially parallel tothe longitudinal axis 17 of the waveguide. It has been found that such aconfiguration produces a substantial improvement in the figure of meritF over that which may be obtained by using materials whose crystals havenot been properly oriented with respect to the longitudinal axis of thewaveguide.

As another example of the orientation of the crystalline structures offerrite materials in accordance with the in vention, there is consideredhere the use of materials having cubic crystalline structures such asthose shown in FIGS. 3b and 30. In accordance with the invention, inorder to provide an optimum figure of merit for materials of this type,it is necessary to substantially align the cubic crystals with respectto both the longitudinal axis direction 21 and the DC magnetic fielddirection 22 of waveguide 20.

The crystal structure shown in FIG. 3b, for example, represents a cubiccrystalline material 23 having a positive anisotropy. For this material,it is necessary to align a direction (face diagonal) of the cubiccrystal in a direction parallel to the magnetic field direction 22. Forthe crystal shown in FIG. 3b, a [110'] direction is designated by thearrow 26 and corresponds to a face diagonal 27, as shown. In additio'mfor this material to be aligned for a maximum figure of merit, it isnecessary to align a [100] direction (cube edge) of the crystal along aline parallel to the longitudinal axis direction 21 of the waveguide. Inthe crystal structure shown in FIG. 3b, a [100] direction is designatedby arrow 28 and corresponds to an edge 29 of the crystal. Ifthe crystalswithin a material of this nature are so aligned, themagnetizationdirection of the material is forced to precess in the correct directionfor optimum isolation operation.

If a cubic crystalline material having a negative anisotropy is used, itis necessary to align the axes as show-nin FIG. 3c. In that figure thereis shown a cubic crystalline material 24 having a [110] face diagonaldirection denoted'by arrow 30 which is aligned with the magnetic fielddirection 22. In addition, another [110] face diagonal direction denotedby arrow 31 is'aligned along the direction of'the longitudinal axis 21of waveguide 20.

In both cases, for FIGS. 3a, 3b and 3c, the materialisoriginally'graimoriented so that the crystals are substantially alignedwith each other within the material along substantially the'samedirection so'that the material may be properly cut to provide'acorrectly oriented slab for use in the configuration shown withrespectto waveguides 10, 'as shown in FIGS. 1 and 2.

.A theoreticalanalysis from a mathematical viewpoint offers some insightinto the reasons why such a grainoriented material provides a betterfigure of merit than that foundin previously knownnon-oriented ferritematerials. The figure of meritfor a non-oriented material utilized in aconfiguration corresponding to FIG. 1 may be expressed in accordancewith the following equation:

In the above equation, M'is the saturation magnetization, w is theoperating frequency, 7 is the gyroma'gneti'c ratio and a may beexpressed in accordance with the following equation:

where w is the cut-ofi frequency of the waveguide.

As a particular example, one may consider the case wherein the operatingfrequency is equal to 2800 megacycles and 21rM is equal to 1000 gauss.In order to provide desirable single mode operation a is, for practicalpurposes, equal to or less than the /3. If a is assumed equal to /3 andif m and 21rM take on the above values, the figure of merit iscalculated as approximately 3.5 for a non-oriented material.

The same formula as expressed above for the figure of merit in theconfiguration of FIG. 1 may be applied to the case of a non-orientedmaterial utilized in accordance with the configuration of FIG. 2 if theterm 21M .is replaced by the term 21rMN where N is the transversedemagnetizing factor. Hence, Equation 1 for the figure of merit may'nowbe written as:

a a a w 2TMN The best figure of merit is obtained when N is very small.Under the same setof conditions given for the example above, the figureof merit for a non-oriented material utilized in FIG. 2 is nowcalculatedas approximately equal to 5.75, or 65 percent'larger than the figure ofmerit' for a non-oriented material calculated previously for thegeometry of FIG. 1. The major difference between the two geometricsituations shown .in FIGS. 1 and 2 liesin the fact that in FIG. 2 thefree precession of the mag- 4 netization vector follows a circular conewhile in FIG. 1, the circle is distorted by the transverseddemagnetizing field into an ellipse whose major axis lies in the planeof the slab, that is to say, in the direction of propagation along thelongitudinal axis of the Waveguide.

Since the figure of merit is larger in the so-called H- plane geometryin FIG. 2 than in the E-plane geometry of FIG. 1, it is suggested that afurther increase in the figure of merit can be obtained by forcing thefree precession of the magnetization vector of a crystalline material tofollow an ellipsoidal cone with its major axis oriented to beperpendicular to the direction of propagation. As shown with respect toFIG. 3a, this orientation can be obtained with a material havinghexagonal crystal structure provided the first-order anisotropy constantis negative. In this case, as explained above, the orientation is suchthat the hexagonal axis substantially coincides with the longitudinalwaveguide axis as shown in FIG. 3a. In the case of cubic crystallinestructures, the orienttion is such that the direction of the magneticfield H coincides with a [110] direction. In addition, if the firstordercubic anisotropy constant is positive, a.[] direction is aligned withthe longitudinal waveguide axis and, if the first order cubic anisotropyconstant is negative, a direction is aligned with the longitudinalwaveguide axis. These conditions are shown, respectively, in FIGS. 3band 30.

Analysis of-the use of grain-orientation such as that which is obtainedby correctly aligning a hexagonal crystalline structure of FIG. 3a showsthat the figure of merit F can be expressed in accordance with thefollowing equation:

In this equation, H is defined as the equivalent anisotropy field andthe other symbols are as defined above.

If (H -411'MN) is much larger than (211- 'Y the ultimate figure of meritis equal to:

For'the value of "a equal to V; F is calculated to have 'a'theoreticalmaximum of 144 which is approximately 25 times better than the bestprevious figure-of merit of 5.75, obtained with non-oriented materials.In a practical sense it is difiicult to obtain this ultimate figure ofmerit because the internal magnetic field necessary to produce resonancedecreases towards zero as the optimum condition for F is approached.Eventually the field is reduced to such a point where it is not strongenough to magnetize the material. However, as one example of apracticable configuration that can be constructed, let us consider ahexagonal crystalline structure wherein 41rM=20UO gauss, N= H,,=2000oersteds, F=2800megacycles and a= For such a configuration the figure ofmerit is 17. If H is made equal to 3000 oersteds and everything elseremains unchanged, the figure of merit F is increased to 27.5. Thus, itcan be seen that the use of grain-oriented materials provides a figureof merit which allows an optimum bandwidth and an optimum reverse toforward attenuation ratio many times that found with 'non orientedmaterials.

The invention is not to be construed as limited to the specificconfigurations shown and described above inasmuch as variations'withinthe scope of the invention will occur to those skilled in the art.Hence, the invention should not be limited except as defined by theappended c aims.

What is claimed is:

1. In combination, a rectangular waveguide for propagating energy in adirection along the longitudinal axis of said waveguide, a ferritematerial positioned Within said waveguide, means for applying a D.-C.magnetic field in a selected direction perpendicular to the direction ofpropagation, said material comprising a plurality of cubic crystalshaving a positive anisotropy constant, the axes of said crystals beingsubstantially aligned with each other within said material, saidcrystals being oriented within said Waveguide so that -a [110] directionof said crystals is substantially parallel to the direction of saidapplied magnetic field and a [100] direction of said crystals issubstantially parallel to the direction of the longitudinal axis of saidwaveguide.

2. In combination, a rectangular Waveguide for propagating energy in adirection along the longitudinal axis of said waveguide, a ferritematerial positioned within said waveguide, means for applying a D.-C.magnetic field in a selected direction perpendicular to the direction ofpropagation, said material comprising a plurality of cubic crystalshaving a negative anisotropy constant, the axes of said crystals beingsubstantially aligned with each other within said material, saidcrystals being oriented within said waveguide so that a first [110]direction of said crystals is substantially parallel to the direction ofsaid applied magnetic field and a second [110] direction of saidcrystals is substantially parallel to the direction of the longitudinalaxis of said waveguide.

References Cited in the file of this patent UNITED STATES PATENTS2,820,200 Du Pre Jan. 14, 1958 2,883,629 Suhl Apr. 21, 1959 2,922,125Suhl Jan. 19', 1960 2,948,870 Clogston Aug. 9, 1960 2,949,588 Weiss Aug.16', 1960 OTHER REFERENCES Weiss: 1955 IRE Convention Record-Part 8,pages -99.

1. IN COMBINATION, A RECTANGULAR WAVEGUIDE FOR PROPAGATING ENERGY IN ADIRECTION ALONG THE LONGITUDINAL AXIS OF SAID WAVEGUIDE, A FERRITEMATERIAL POSITIONED WITHIN SAID WAVEGUIDE, MEANS FOR APPLYING A D.-C.MAGNETIC FIELD IN A SELECTED DIRECTION PERPENDICULAR TO THE DIRECTION OFPROPAGATION, SAID MATERIAL COMPRISING A PLURALITY OF CUBIC CRYSTALSHAVING A POSITIVE ANISOTROPY CONSTANT, THE AXES OF SAID CRYSTALS BEINGSUBSTANTIALLY ALIGNED WITH EACH OTHER WITHIN SAID MATERIAL, SAIDCRYSTALS BEING ORIENTED WITHIN SAID WAVEGUIDE SO THAT A (110) DIRECTIONOF SAID CRYSTALS IS SUBSTANTIALLY PARALLEL TO THE DIRECTION OF SAIDAPPLIED MAGNETIC FIELD AND A (100) DIRECTION OF SAID CYRSTALS ISSUBSTANTIALLY PARALLEL TO THE DIRECTION OF THE LONGITUDINAL AXIS OF SAIDWAVEGUIDE.